Oh, nice one. I wouldn't be a bit surprised if the filename 'text.epub' is hard-coded to test this situation. Have you run the file through FlightCrew? Also, try renaming it to 'text1.epub'. I think it will work then...

Nope, that's not it. I uploaded a 'text.epub' and it validated without problems. Tried renaming the file? Is there a file in the container with a 'special' character in the name? Perhaps a hidden Apple name/file?